Add Kustomize detector line mapping

[whitemerch]

Tip

This PR is best reviewed commit by commit.

Summary

• add a dedicated Kustomize detector that maps findings back to direct sources, generator entries, and transformer patch references
• add helper test wiring for subprocess kustomize rendering during detector package tests
• connect detector mapping to vulnerability-line attribution in the engine path

Full design context and architecture details: #113

Test plan

• go test ./pkg/detector/kustomize/... ./pkg/engine/... -count=1
• $(go env GOPATH)/bin/golangci-lint run ./pkg/detector/kustomize/... ./pkg/engine/...

Notes

• stacked on top of PR Add Kustomize resolver preprocessor core #115
• scan activation and fixtures remain in follow-up PRs

[datadog-datadog-prod-us1]

🎯 Code Coverage (details)
• Patch Coverage: 53.39%
• Overall Coverage: 47.10% (+0.30%)

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